Heretofore, the antireflection treatment of optical parts has been mainly carried out by a method which comprises depositing an inorganic substance (such as metal oxide or fluoride) in the form of a film on the surface of a given optical part by the so-called dry plating technique such as vacuum deposition, spattering, or ion plating.
Since the method of this nature requires the treatment to be carried out under a vacuum, it imposes a limit on the size of the substrate and suffers the treatment to become expensive. By this reason, the optical parts which are now benefiting from the antireflection treatment are limited to relatively expensive products of high added value. Virtually all general-purpose products of quantity manufacture are not given this treatment.
For the optical parts using inorganic glass substrates, therefore, a method which comprises etching the surface of a given glass substrate with an aqueous solution of hydrosilicofluoric acid or hydrofluoric acid supersaturated with silica as disclosed in U.S. Pat. No. 2,490,662 and U.S. Pat. No. 4,535,026 and a method which comprises superposing a multicomponent glass layer on the surface of a given substrate through a sol-gel process and etching the glass layer with hydrofluoric acid as disclosed in U.S. Pat. No. 4,273,826 have been proposed. In Japanese Utility Model Publication SHO 50(1975)-26,277 discloses a method which produces a braun possessing an antireflection effect by spraying silicon tetrachloride on the surface of a Braun tube and thermally setting the deposited layer of silicon tetrachloride thereby giving rise to very minute silica particles.
These methods permit the antireflection treatment to be carried out inexpensively. They, however, have a disadvantage that they are incapable of giving an effective antireflection treatment to plastic substrates and the antireflection films produced thereby are inferior in strength of film and density of film, namely in wear resistance and scratch resistance, to those produced by the dry plating methods mentioned above, and they are liable to be defiled when they are handled with bare hands.
On plastic substrates, the antireflection treatment can be effectively carried out by (1) the film superposition method by means of wet immersion (based on the same principle as the aforementioned dry plating method) as disclosed in Japanese Patent Publication SHO 58(1983)-46,301 and SHO 59(1984)-49,501 and (2) the so-called coating method which makes use of a treating solution containing colloid particles such that, when the solution is applied in the form of a film on the surface of a given substrate, the colloid particles give rise to fine irregularities on the surface as disclosed in Japanese Patent Application Disclosure SHO 58(1983)-126,502, for example.
In the case of the method of (1) mentioned above, uniform superposition of the film in two or three layers by means of immersion (including the control of film is extremely difficult to achieve where the given substrate has a large size or involves a large curvature. The method, therefore, the liable to entail a problem of production control. The inventor, therefore, has found a special interest in the method of (2) which permits easy production control.
The colloid particles involved in the method of (2) are generally formed mainly of the hydrolyzate (including polycondensate) of a silicic ester. Since the colloid particles of a fixed diameter are sporadically deposited on the coated surface, they manifest their function as an antireflection medium by lowering the apparent refractive index of the glass substrate. This method, however, has been found to entail a disadvantage that since mutual polymerization of individual colloid particles does not readily proceed at the specified low temperature (80.degree. to 130.degree. C.), the produced film possesses low strength, tends to collect dirt (because of surface irregularities), and betrays poor adhesion to the substrate as compared with the film produced by the aforementioned dry plating method.